You must ride a Husky or a KTM?
You didn't say which 400cc "dirt bike" you're traveling on, so above is just a guess.

Some smaller bikes certainly are real dirt bikes, others simply dual sport bikes that need modification to suit touring. Some can be modified, some aren't worth the effort/cost. But as you say, the pay off is when things get nasty. But of course a two way street on that front.

Even my brand new 125cc Honda two stroke CRM rental got bogged down in nasty red clay mud in N. Thailand. Can't imagine something heavier.

Historically, one of the most popular 400cc "dirt bikes" is not really a dirt bike at all. Talking Suzuki's DRZ400S. (include DRZ400E which used to be sold in UK.) E model was Better dirt bike, less good travel bike than S model. (I owned the "E" model, never went beyond Baja and California, great dirt bike!)

Many have done extensive mods on the DRZ400S and done RTW travel. Wide seat, proper tires for road work, small screen. I found stock gearing on that DRZ-S was good for 85 mph measured. Not bad.

Most of the guys who rode the Suzuki's have sold them on and bought either Husky or KTM 350's or 500's EXC's.

They have their own set of problems ... like longevity. Those old DRZ's could easily rack up 30K to 40K miles ... the Austrians are usually in need of service by 15 to 20K miles. More compromises.

And the Euro bikes are SUPER tall, whereas the old DRZ is reasonable for most.